CHARLOTTE, N.C. (Reuters) - U.S. farm groups were turning critical of the government's handling of mad cow disease on Sunday, with the largest ranchers' group fearing a drop in cattle prices.

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Others are calling for the largest U.S. farm group to oppose a Bush administration plan to accept cattle from Canada, which just found a new case of the disease.

Critics said it's unwise to open the border with Canada without first restoring U.S. beef sales to Japan and South Korea (news - web sites ), traditionally two major customers who cut off sales after the first U.S. case of the fatal bovine disease was found a year ago.

The criticism was a loud counterpoint to the often-stated argument the United States, by setting a precedent with Canada for trade with nations battling mad cow, could unlock the Japanese and South Korean markets. U.S. beef exports are forecast to be a small fraction of their usual lucrative level because major buyers shun the meat.

Without exports, cattle prices could fall under the weight of Canadian cattle imports, skeptics said. At the same time, U.S. meatpackers say they cannot operate at full capacity without them and have laid off some workers.

Under an Agriculture Department plan, imports of Canadian cattle less than 31 months old would resume in March. The list of beef items that can be imported also would expand.

"We would like to see them delay the opening (of the border) at least until early summer, June or July," said Steve Kouplen, president of the Oklahoma Farm Bureau, at the opening of the annual meeting of the 5.6 million-member American Farm Bureau (news - web sites ) Federation.

At present, AFBF supports the Bush administration approach. Kouplen said his delegation would propose a change in policy later this week. "It's not strictly economics," he said. "We're concerned with consumer confidence."

Americans eat as much beef as before mad cow was discovered in North America. U.S. officials say safeguards against the disease are so strong in Canada and the United States that the rare discovery of an infected animal is not grounds to cancel trade. USDA officials say Canada could report up to 11 cases of the fatal bovine disease and still be classed as a minimal-risk nation.

"It's always important to remember (that) how we treat our imports will affect how other nations treat our exports," said AFBF President Bob Stallman.

Stallman said "we do support" the March border opening. But he also called for congressional oversight of the plan. "It's vital to get this right."

Long a staunch ally of President Bush (news - web sites ), the National Cattlemen's Beef Association (news - web sites ) said on Friday that it "will insist by March 7 trade be re-established with Japan and South Korea and expanded in Mexico through negotiations between the highest levels of government officials or further action will be taken."

That was sharply different from its previous statement, "We must normalize trade with Canada in order for our industry to move forward in the global marketplace, expand our ability to sell U.S. beef to foreign consumers and put more dollars in the pockets of U.S. producers."

An NCBA official said the March 7 border opening underlined the need to protect cattle producer income. "How much is this going to cost us?" said NCBA official Jay Truitt. "The administration has to jump on (beef-)exporting possibilities."

The 300,000-member National Farmers Union and the R-CALF USA United Stockgrowers of America opposed the border opening.